1. Field of the Invention
The present invention relates generally to automatic transmissions and more specifically to a control arrangement for a transmission which has main and auxiliary gear trains and which facilitates the reduction of shift shock.
2. Description of the Prior Art
JP-A-2-51657 discloses an automatic transmission which includes a main four speed main gear train and an auxiliary gear train which can be selective switched between high and low gears. In combination, the main and auxiliary gear trains are arranged to produce 5 forward speeds and one reverse. In order to produce the lowest gear of the main gear train, use is made of a first one-way clutch.
The auxiliary transmission includes a clutch, a brake and a second one-way clutch. When the clutch is engaged the planetary gear assumes a directly engaged condition and produces a speed change ratio of 1. When only the clutch is engaged the auxliary gear train is condition to produced direct drive while when the either of the one-way clutch or the brake is engaged a reduction gearing is established.
In the case the one-way clutch is exposed to torque which is produced from the engine it locks and transmits torque. On the other hand, in the reverse instance wherein the output shaft of the transmission is induced to rotate faster that the rotation which would be induced by the engine (e.g. the vehicle is coasting) the one-way clutch free-wheels. The brake is engaged only when engine braking is required and normally (e.g. D range conditions) the one-way clutch is used to transmit torque.
In first second and third speeds the auxiliary gear train is conditioned to produce the reduction gearing, while the actual shifting takes place in the main gear train. When the transmission is conditioned to produce third speed the main gear train produces a gear ratio of 1. Following this the auxiliary gear train is conditioned to produce direct drive (a gear ratio of 1) and establish fourth speed. Next, while the auxiliary gear train is conditioned to produce direct drive, the main gear train is shifted to produce its top speed. This results in the transmission being conditioned to produce fifth speed (overdrive).
However, with this type of transmission if the engine throttle is fully closed and the transmission undergoes in no (engine) load upshift, a large shift shock is generated.
For example, assume that the transmission is conditioned to produce second speed with the accelerator pedal depressed to a given degree, the accelerator pedal is then released to induce the engine throttle valve to fully close, and the transmission shifts up to third speed. Under these conditions the upshift takes place with the auxiliary gear train conditioned to produce the reduction gearing. However, the torque which is being produced by the engine reduces as a result of the accelerator pedal release. Before the engine output torque reaches a zero level the conditions under which the driving force is transmitted between the engine and the transmission output shaft is such that the change in torque is transmitted to the vehicle chassis and results in the generation of a large shift shock.